Without limiting the scope of the invention, its background is described in connection with heterojunction bipolar transistors, as an example.
Heretofore in this field heterojunction bipolar transistors (HBTs) have attracted a great deal of interest for their potential in both microwave and digital applications. HBT structures have a heterojunction at the emitter-base junction which enables the current gain of the device to be almost independent of epitaxial layer doping concentrations. HBTs can thus be readily optimized for high-speed or high frequency operation by selecting appropriate layer properties, in contrast to typical silicon homojunction bipolar transistors. Although it is very important to minimize transit times and parasitic resistive elements within the active device, high performance operation also demands a minimization of all parasitic external device capacitances. Parasitic elements such as the capacitance between contact pads of the emitter, base, and collector electrodes, as well as the parallel-plate capacitance between emitter or base contact pads and the buried subcollector, must be minimized.
Due to its excellent semi-insulating properties, a GaAs substrate can easily isolate the device contact pads from the ground plane. However, the interelectrode isolation requires either ion implantation or mesa etching to reduce the conductivity of layers beneath the contact metals. Both techniques are commonly used in GaAs devices and are applicable to HBTs as well. Although mesa etching is intuitively the simplest way to achieve isolation, it results in non-planar device topography and limits the potential complexity of circuits. Ion implantation has been applied successfully to HBT fabrication and has shown greater potential for integrated circuits due to resulting planar or quasi-planar surfaces. Proton, oxygen, proton and oxygen, and boron implants have been used in the past. Although implantation is the preferred isolation technique, it can have a deleterious effect on the DC and 1/f noise characteristics of a device depending on process flow, device structure and the type of implant used.